Esterification of cholesterol

HDL particles extract cholesterol from peripheral membranes and, after esterification of cholesterol to a fatty acid, the cholesteryl esters are delivered to the liver (to make bile salts) or steroidogenic tissues (precursor of steroids). 

Apolipoprotein A-II is a component of HDL and VHDL particles, and serves as a cofactor of enzymatic reactions that take part in esterification of cholesterol and mediate its transport. 

Esterification of cholesterol When cholesterol is taken up by HDL, it is immediately esterified by the plasma enzyme phos-phatidylcholine cholesterol acyltransferase (PCAT, also known as LCAT, in which "L" stands for lecithin). This enzyme is synthesized by the liver. PCAT binds to nascent HDLs, and is activated by apo A-l. PCAT transfers the fatty acid from carbon 2 of phosphatidyl-... 

Studies by Bell and coworkers have shown that DEHP also can alter sterologenesis in rodents, which may have an impact on steroid-dependent functions, such as reproductive functions. For example, feeding female rats DEHP at an estimated dose of 500 mg/kg/day for 13 days significantly inhibited sterologenesis from 14C-mevalonate in liver and adrenal minces (Bell 1980). DEHP also inhibited cholesterol synthesis in the liver from male rats and rabbits as well as in rats testes (Bell 1982). In a subsequent study, Bell and Buthala (1983) demonstrated that the inhibition of cholesterol synthesis in the liver was due to a reduction in the activity of microsomal acylCoA cholesterol acyltransferase, an enzyme responsible for the esterification of cholesterol. 

Hll. Haugen, R., and Norum, K. R., Coenzyme-A-dependent esterification of cholesterol in rat intestinal mucosa. Scand. J. Gastroenterol. 11, 615-621 (1976). 

K19. Kostner, G., The influence of various lipoproteins and apolipoproteins on the in vitro esterification of cholesterol in human serum by the enzyme lecithimcholesterol acyltransferase. Scand. J. Clin. Lab. Invest. 38 (Suppl. 150), 66-71 (1978). 

Recently, milk sphingomyelins were reported to interact significantly with the physical state of cholesterol, which correlated positively with reduced uptake and esterification of cholesterol by Caco-2 cells they also significantly reduced cholesterol absorption in mice, even at 0.1% of the diet (Eckhardt et al., 2002). An earlier study showed the regulation of cholesterol absorption by the content of sphingomyelin in intestinal cell membranes (Chen et al., 1992). 

Acyl-CoA cholesterol acyltransferase catalyzes the esterification of cholesterol in the cell cytoplasm ... 

ACAT transfers amino-acyl groups from one molecule to another. ACAT is an important enzyme in bile acid synthesis, and catalyses the intracellular esterification of cholesterol and formation of cholesteryl esters. ACAT-mediated esterification of cholesterol limits its solubility in the cell membrane and thus promotes accumulation of cholesterol ester in the fat droplets within the cytoplasm this process is important in preventing the toxic accumulation of free cholesterol that would otherwise damage ceU-membrane structure and function. Most of the cholesterol absorbed during intestinal transport undergoes ACAT-mediated esterification before incorporation into chylomicrons. In the liver, ACAT-mediated esterification of cholesterol is involved in the production and release of apo-B-containing lipoproteins. 

A proposed mechanism by which an exercise-induced increase in LCAT leads to an increase in HDL cholesterol is illustrated in figure 3. An increase in both Apo AI and LCAT activity in response to exercise could lead to Increased esterification of cholesterol in HDL and thereby allows for an increase in the transport of free cholesterol from tissues and other lipoproteins to nascent HDL, and enhanced formation of HDL2. While still speculative, this proposed mechanism deserves attention. 

It has been suggested that Pgp plays a role in the transport of prcnyleystcin-rue thy 1 esters or cholesterol (58). In addition, esterification of cholesterol and triacylglycerol-rich lipoprotein secretion was inhibited by inhibitors of Pgp (59), whereas cholesterol seemed to be transported by Pgp also (60). 

Figure 26-5 Intracellular and intravascular esterification of cholesterol mediated by ACAT and LCAT, respectively.

The various esters of cholesterol, retinoyl esters, esters of vitamin D and E, probably depend solely on the activity of carboxylester lipase for their hydrolysis. They are, before their uptake by the enterocytes, transformed into the corresponding alcohols to form mixed micelles with bile salts. Carboxylester lipase catalyzes not only the cleavage of esters but also their formation. The most studied example is the reversible esterification of cholesterol, which is favored by low bile salt concentrations and pH [42]. Additionally, the laige specificity of carboxylester lipase probably functions as a first-line detoxification mechanism for a broad variety of orally ingested xenobiotics. 

The bile-salt-dependent lipase of pancreatic juice has many names such as cholesterol esterase, nonspecific lipase, the most rational being carboxyl ester lipase [27], In the case of water-insoluble substrates this enzyme has an absolute requirement for bile salts specifically having hydroxyl groups in the 3a and la positions [28.29]. The best documented role for this enzyme is to allow the absorption of dietary cholesterol, through hydrolysis of cholesterol esters in the lumen. The enzyme also catalyzes the esterification of cholesterol and a role for it has been proposed in cholesterol absorption [30]. In addition, a wide range of primary and secondary fatty acyl esters including glycerides, vitamin A and E esters are hydrolyzed by this enzyme. 

Enzymic processes in the enterocyte which handle a number of lipid species other than triglycerides are also known to be influenced by bile salts. For example, it has been postulated that pancreatic cholesterol esterase is absorbed by the mucosal cells and catalyzes the esterification of cholesterol in the cell [87]. This postulate has been challenged by Watt and Simmonds [88] although Bhat and Brockman [30] recently produced evidence for the importance of pancreatic cholesterol esterase for transport across the enterocyte membrane. Another enzyme, acyl-CoA cholesterol acyltransfera.se (ACAT) of microsomal origin, has been ascribed a major role in mucosal esterification of cholesterol [89], Unlike pancreatic cholesterol esterase, this enzyme is inhibited by taurocholate in vitro. 

Hydrolysis of triglycerides and phospholipids by lipases and esterification of cholesterol by an acyl transferase... 

Cholesteryl esters are quantitatively minor constituents (5-15% of total lipids) of VLDLs but the amount of cholesteryl esters relative to TG in VLDLs increases when rats are fed a high cholesterol diet. The esterification of cholesterol is mediated by two distinct acyl-CoA cholesterol acyltransferases (ACATs) [11]. Inhibition of cholesterol esterification with an ACAT inhibitor in hepatocytes decreased apo B secretion in some studies but not in others. For example, severe reduction in cholesteryl ester content of hepatoma cells decreased apo B secretion, whereas increased cholesteryl ester content did not stimulate apo B secretion. In mouse liver and intestine, the majority of cholesteryl esters are made by ACAT2. Nevertheless, normal quantities of apo B-containing lipoproteins are produced in mice lacking ACAT2 despite the absence of essentially all hepatic ACAT activity. However, ACAT2-deficient mice exhibit reduced intestinal absorption of cholesterol and are resistant to diet-induced hypercholesterolemia (R.V. Farese, 2(X)0). Thus, the observed reduction of plasma cholesterol in response to ACAT inhibitors is probably due to decreased cholesterol absorption rather than decreased VLDL secretion. 

Phosphatidylcholine is the donor of the acyl group to cholesterol in the LCAT-catalyzed esterification of cholesterol. 

Inhibitors of acyl CoA-cholesterol acyltransferase (ACAT) are currently being Investigated as cholesterol-lowering or antiatherosclerotic agents. In addition to its role in foam cell formation, ACAT also is required for esterification of cholesterol in intestinal mucosal cells and for synthesis of cholesterol esters in hepatic VLDL formation. Thus, ACAT inhibitors have the potential of providing three beneficial effects in patients with hypercholesterolemia decreased cholesterol absorption, decreased hepatic VLDL synthesis, and decreased foam cell formation. Initial successes at inhibiting ACAT were dampened by the discovery of accompanying adrenal toxicity. Subsequent structural modifications have lead to the development of... 

The esterification of cholesterol in animals has attracted considerable research because of the possible involvement of cholesterol and its ester in various disease states (cf. Glomset and Norum, 1973, and Sections 12.1, 12.3 and 12.6). Cholesterol esters are formed by the action of lecithin cholesterol acyltransferase (LCAT, EC 2.3.1.43) which is particularly active in plasma (cf. Sabine, 1977, for a review of cholesterol metabolism). The reaction involves transfer of a fatty acid from position 2 of lecithin (phosphatidylcholine) to the 3-hydroxyl group of cholesterol with the formation of monoacyl-phosphatidylcholine. Although LCAT esterifies plasma cholesterol solely at the interface of high-density lipoprotein and very-low-density lipoprotein, the cholesterol esters are transferred to other lipoproteins by a particular transport protein (CETP cholesteryl ester transfer protein). Cholesteryl esters, in contrast to free cholesterol, are taken up by cells mostly via specific receptor pathways (Brown et aL, 1981), are hydrolysed by lysosomal enzymes and eventually re-esterified and stored within cells. LCAT may also participate in the movement of cholesterol out of cells by esterifying excess cholesterol in the intravascular circulation (cf. Marcel, 1982). 

Fig. 4. Esterification of cholesterol catalyzed by lecithin-cholesterol acyltransferase (LCAT).

The mode of action of sitosterol, which differs in structure from cholesterol by possession of an ethyl group on C-24, has not been established. In contrast to cholesterol, intestinal absorption of sitosterol has been shown to take place in only very small quantities (Schoenheimer 1929,1932, Best 1956, Schettler 1961) and amounts to about 5% of administered sitosterol, (Gould 1955). According to one theory sitosterol interferes with the absorption of cholesterol. If cholesterol and sitosterol are administered simultaneously, the absorption of the former is markedly decreased (Hernandez et al. 1953) only one-third of the cholesterol is absorbed if both substances are administered in equal parts, while cholesterol absorption is nil when cholesterol and sitosterol are fed in the proportion of 1 7 (Pollak 1953). Mixed crystal formation may be responsible for this effect, as suggested by the in vivo and in vitro studies of Davis (1955) and of Hudson and co-workers (1959), who found crystals with an X-ray diffraction pattern different from either cholesterol or sitosterol, and suspected the presence of a less-dispersible compound. The assumption of Swell et al. (1954) that there is competitive inhibition of esterification of cholesterol by sitosterol, has been refuted by Blomstrand and Ahrens (1958), and the suggestion of Glover et al. (1957) of competition for acceptor lipoproteins is unproved. Gerson and Shorland (1963), on the basis of isotopic studies in rats, considered the effect of beta-sitosterol on cholesterol absorption to be less important, and discussed the effects of the sterol on cholesterol metabolism and on the cholesterol content of different tissues. 

The esterification of cholesterol is considered by Nieft and DeueF and by Saviano and BaccarF to be due to an enzymatic activity separate from that which hydrolyzes cholesteryl esters. With but one exception the hydrolysis of cholesteryl esters has taken place in the same extract as that in which esterification of cholesterol occurred, and at a higher pH than esterification.. 72.74 jg difficult to reconcile the concept of the true... 

This fundamental role of bile salts in the intestinal absorption of sterols is a reflection of the potential requirements for this cholesterol metabolites in various steps of intraluminal and epithelial cell mechanisms of cholesterol absorption (Figure 1), These include solubilization of cholesterol in the intestinal lumen by mixed micelles, containing biliary bile salts and phospholipids, and the products of triglyceride digestion modification of the intestinal surface barriers to cholesterol transfer, including the un-stirred water layer" and the mucin "coat" and the cellular esterification of cholesterol prior to incorporation of the resulting esters into the lipoprotein core lipids. 

The cellular esterification of cholesterol prior to efficient translocation into lymph in the chylomicron and VLDL core[53] has been considered by many to be the limiting step in absorption. This is based on a variety of physiological and pharmacological evidences (see[ll-15j. It is at this site that a unique requirement for cholic acid has been demonstrated in the rat (Table 3). 

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